A standard prestressed concrete beam comprises an elongated body of concrete in which is imbedded a plurality of longitudinally extending and normally throughgoing prestressing members that are normally under tension. The members are typically steel rods in the form of wires or cables.
One standard method of making such a beam entails fitting the rods through an elongated upwardly open form with the ends of the rods projecting from the ends of the form. Nuts or other traction elements are applied to the projecting ends and are tightened to put the rods under tension. The form is then filled with concrete which is screeded off level with its rim, and the mass is allowed to cure. Once sufficiently hard, the traction elements are released and the thus formed beam is demolded and the rods are cut off flush with the end of the form. Alternately the rods can end short of the end of the form and be held by releasable traction elements so as not to project past the form end. In this system, which is described in German utility model 1,833,644, ribbed reinforcement rods are used so that they bond along their full lengths to the surrounding concrete and, as a result, the tension in the rods is transferred to the entire beam.
This system has the disadvantage that only a limited number of rods can normally be clamped due to the space occupied by the traction nuts, so that certain types of heavy-duty beams cannot be produced. Furthermore the beam must remain in the mold until it is fully cured, otherwise the rod will retract and separate from the surrounding concrete.
In another known method described in German utility model 1,744,448 the group of rods is fitted through a pair of plates formed with an array of holes each receiving and slightly larger than the respective rod. The plates are arranged at the ends of the group of rods and the rods are formed with heads that bear inward against the plate. This assembly of rods and plates is set in the mold and each plate is attached to a traction element that extends through the end wall of the mold, traction is applied to tension the rods, and the mold is filled with concrete. When the concrete has substantially cured, the traction elements, which can be simple bolts threaded into the plates, are withdrawn and the beam is demolded. Such a system can use relatively smooth reinforcing rods as the prestress force is not transmitted to the concrete as much by adherence of the concrete to the reinforcement rods as by the plates in which the headed ends of the rods are seated.
The main disadvantage of this system is that the anchor plates must remain in the finished beam. This plate represents a not negligible cost that is reflected in the price of the finished product. When same is a mass-produced item like a railway sleeper, the additional cost of these anchor plates adds noticeably to the price.